Podcast
Questions and Answers
The chemical shift range for alkyl hydrogens (C-H) is ______ ppm.
The chemical shift range for alkyl hydrogens (C-H) is ______ ppm.
0.5–2.0
In an NMR spectrum, a CH3 group in ethanol appears as a ______ due to its coupling with the CH2 group.
In an NMR spectrum, a CH3 group in ethanol appears as a ______ due to its coupling with the CH2 group.
triplet
The chemical shift for aldehyde hydrogens (C-H=O) is typically found between ______ ppm.
The chemical shift for aldehyde hydrogens (C-H=O) is typically found between ______ ppm.
9.0–10.0
To analyze NMR spectra, one must look at the number of peaks, chemical shifts, and ______ patterns.
To analyze NMR spectra, one must look at the number of peaks, chemical shifts, and ______ patterns.
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An ester functional group can be identified in an IR spectrum by the presence of a ______ peak.
An ester functional group can be identified in an IR spectrum by the presence of a ______ peak.
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In 13C NMR, each peak represents one set of equivalent ______ atoms.
In 13C NMR, each peak represents one set of equivalent ______ atoms.
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In 1H NMR, peak intensity indicates the number of equivalent ______ atoms.
In 1H NMR, peak intensity indicates the number of equivalent ______ atoms.
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Solvents used in NMR must not contain ______ atoms to avoid interference.
Solvents used in NMR must not contain ______ atoms to avoid interference.
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TMS, or ______, is used as a standard for calibrating NMR spectra.
TMS, or ______, is used as a standard for calibrating NMR spectra.
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Chemical shift (δ) is a measure of how much the magnetic field experienced by a nucleus differs from ______.
Chemical shift (δ) is a measure of how much the magnetic field experienced by a nucleus differs from ______.
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In 1H NMR, neighboring inequivalent H atoms can cause ______, leading to peak splitting.
In 1H NMR, neighboring inequivalent H atoms can cause ______, leading to peak splitting.
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A singlet in NMR indicates there are ______ neighboring H atoms.
A singlet in NMR indicates there are ______ neighboring H atoms.
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The relative intensity ratios for a triplet in NMR is ______:2:1.
The relative intensity ratios for a triplet in NMR is ______:2:1.
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Study Notes
NMR Spectroscopy
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Types of NMR:
- 13C NMR: Detects carbon atoms. Only a small percentage of carbon atoms are 13C
- 1H NMR: Detects hydrogen atoms. The intensity of peaks relates to the number of hydrogen atoms.
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Solvents:
- Solvents without hydrogens are essential.
- Examples include CCl4 (non-polar) and CDCl3 (polar)
- CDCl3 gives a characteristic peak
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TMS (Tetramethylsilane):
- Standard for calibration in both 1H and 13C NMR.
- Single, sharp peak, far from organic component peaks.
- Non-toxic
- Low boiling point
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Chemical Shift (δ, ppm):
- Measures the difference between the magnetic field experienced by a nucleus and TMS.
- Measured in parts per million (ppm).
- Electronegative atoms increase shift (deshielding).
13C NMR Chemical Shift Ranges
- 5-40 ppm: Alkanes (C-C or C-H bonds).
- 50-90 ppm: C bonded to O.
- 160-185 ppm: Carbonyl carbons (C=O).
1H NMR Key Concepts
- Equivalent Hydrogens: Same signals in the spectrum.
- Intensity (Integration): Area under a peak= number of equivalent hydrogens.
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Spin-Spin Coupling (Splitting):
- Neighboring inequivalent hydrogens cause splitting.
- Splitting rule: Number of peaks = Number of neighboring H’s + 1.
- Example: 3 neighboring hydrogens ⇒ Quartet (4 peaks)
1H NMR Splitting Patterns
- Singlet (1 peak): No neighboring hydrogens.
- Doublet (2 peaks): 1 neighboring hydrogen.
- Triplet (3 peaks): 2 neighboring hydrogens.
- Quartet (4 peaks): 3 neighboring hydrogens.
1H NMR Integration Ratios (Pascal's Triangle)
- Doublet: 1:1
- Triplet: 1:2:1
- Quartet: 1:3:3:1
1H NMR Chemical Shift Ranges (ppm)
- 0.5-2.0 ppm: Alkyl hydrogens (C-H).
- 2.0-3.0 ppm: Hydrogens near electronegative groups(ex: C=O,C≡C)
- 3.0-4.5 ppm: Hydrogens bonded to C-O.
- 4.5-6.5 ppm: Hydrogens in alkenes (C=C).
- 6.5-8.0 ppm: Hydrogens in aromatic rings.
- 9.0-10.0 ppm: Aldehyde hydrogens (C-H=O).
- 10.0-12.0 ppm: Hydrogens in carboxylic acids (O=C-O-H).
Example: Ethanol (CH3CH2OH)
- CH3 (a): Triplet (2 neighboring H's; 0.7–1.2 ppm, integration = 3)
- CH2 (b): Quartet (3 neighboring H's; 3.7–4.1 ppm, integration = 2)
- OH (c): Singlet (no coupling; 0.5–5.0 ppm, integration = 1)
NMR Analysis Steps for Identifying Compounds
- Determine the Empirical Formula (elemental analysis).
- Find the Molecular Formula (mass spectrometry).
- Identify Functional Groups (IR spectroscopy).
- Analyze NMR Spectra (peak numbers, shifts, splitting).
Example: Compound Analysis (C8H16O2 Ester)
- Molecular formula: C8H16O2
- IR spectrum: C=O peak, no O-H peak ⇒ ester
-
1H NMR peaks:
- δ = 0.9 ppm (Singlet, area 9) ⇒ 3 equivalent CH3 groups.
- δ = 4.0 ppm (Quartet, area 2) ⇒ CH2 group adjacent to CH3.
- δ = 2.2 ppm (Singlet, area 2) ⇒ CH2 group adjacent to C=O.
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Description
This quiz covers the fundamentals of NMR spectroscopy, focusing on 1H and 13C NMR techniques. You'll learn about the types of NMR, the importance of solvents, TMS calibration, and key concepts like chemical shifts. Test your understanding of how these principles apply to molecular structure analysis.
